Source-sink dynamics may define the patterns in distribution and abundance of many marine and estuarine species. In source habitats there is a demographic surplus (births + emigration > deaths + immigration), whereas in sinks a demographic deficit (deaths + immigration > births + emigration) leads to local extinction, without immigration from sources. Moreover, total production in linked sources and sinks may be higher than in either alone, due to subsidies to sinks by emigrants from sources. Hypotheses based on source-sink dynamics implicitly incorporate recruitment processes, physical transport mechanisms, dispersal behavior, post-settlement demography, and habitat heterogeneity across various scales of space and time.
Many of the current paradigms of marine ecology can be subsumed or applied within the conceptual framework of source-sink dynamics. In this project, we use extensive population data and a stage-structured, spatially-explicit matrix population model with dispersal to test hypotheses concerning survival, growth, reproduction, and dispersal between subpopulations in putative source and sink habitats for the Baltic clam Macoma balthica. The sources and sinks have been tentatively identified for Macoma in two independent, spatially separated and environmentally different locations in Chesapeake Bay—the Rhode and York Rivers.
We hypothesize that there exists a system of vital source habitats (shallow mud flats) and linked sink habitats (adjacent sand flats; detrital muds) for Macoma that dictate its persistence and population fluctuations. The nominal sources are hypothesized to contribute significantly more to the recruit pool than sinks; recruitment into sinks from the recruit pool is disproportionately higher than their reproductive output. Food limitation may be the primary mechanism stimulating redistribution of recruits (post-larvae and young juveniles) between sources and sinks. If emigrants from sinks re-invade source habitats, then total production in the population is enhanced through source-sink dynamics. We posit that this represents the first comprehensive test of the existence and consequences of source-sink dynamics in a marine system.